MPs are grappling with the intricacies of the British constitution this week as they debate a shake-up of the succession laws.
Most attention has focused on the end of discrimination against female heirs, meaning Kate and William's first-born gets to wear the crown - even if she is a girl.
But there have also been rumblings of discontent - notably from some Tory backbenchers and the DUP - about another provision in the Bill: the end of the ban on heirs to the throne from marrying a Catholic.
The bar on a Catholic monarch remains, but rules preventing a monarch being married to a Catholic are to be lifted.
Concern has focused on the possible side-effects. What would happen were the children of that couple to be raised as Catholics - given the monarch's role as head of the Church of England?
As the Succession to the Crown Bill gets its second reading, North Antrim MP Ian Paisley jnr will attempt to tweak the legislation to ensure there are no consequences for the monarch's role as head of the Church of England.
His amendment would ensure that anyone in line to the throne "remains in communion with the Church of England", in line with the Act of Settlement.
He is supported by his party and several Tory MPs.
Mr Paisley argued that the line of succession had served the country well and said the planned changes could open up a "royal Pandora's box".
He referred to the opposition as a "papal prod pleb alliance". He said he hoped such an alliance would be "listened to by this house".
Deputy Prime Minister Nick Clegg yesterday told MPs: "The current rules of succession belong to a bygone era. They reflect old prejudices and old fears. "
He also told the Commons: "Today we don't support laws which discriminate on either religious or gender grounds.
"They have no place in modern Britain and certainly not in our monarchy - an institution central to our constitution, to the commonwealth and to our national identity too."
Conservative backbencher Jacob Rees-Mogg, one of Parliament's authorities on constitutional matters, believes the Government's proposals are "nonsensical". A Catholic, he points to his duty under Canon Law to raise his children in the same way.
His amendment would allow a Catholic to take the throne, but the Church role would transfer to someone else.
He believes this is the only way to avoid a constitutional mess, but was not optimistic that either he, or Mr Paisley, could win the Commons vote.
This all follows concerns raised by a House of Lords committee, which said the monarch could be asked to veto an heir's marriage of in order to protect the succession.
The Government says it has no plans to change the relationship between Church and state.
Deputy Prime Minister Nick Clegg has insisted there is "a lot of flexibility" in the Catholic Church on how children are raised.
And spare a thought for hereditary peers, subject to similar gender discrimination as the Royals.
But the Government isn't bothering with them, so barons who only have daughters are forced to leave their vast estates to their nephews.
The proposed changes also need to be approved by the 15 other countries of which the Queen is head of state.
They agreed in principle to do so in 2011. The bill is expected to return to the house next week for further debate.